(1) Field of the Invention
This invention relates to projection lithography systems for imaging onto curved substrates, and more particularly relates to a large-area lithography system featuring a curved mask that is identical in size and shape to the curved substrate. An axially moving 1:1 projection lens achieves a constant optical path length for conjugate image points in order to maintain the substrate surface within the depth-of-focus, thereby providing an effective depth-of-focus much larger than the depth-of-focus of the projection optics itself. This invention is centered around a novel illumination compensator which we call ‘Zerogon’, that is part of an illumination system and protects the converging illumination beam from various image anomalies when it transmits through a curved mask. This unique optical system with curvatures on its elements has zero power and works like an un-tilted plane glass blank in the path of a given collimated or convergent beam. A detailed paraxial ray theory was developed to demonstrate the functionality of such a device. Two possible configurations for Zerogon have been described in the invention. The unique device facilitates patterning on curved surfaces by means of small-field seamless scanning techniques to achieve high resolution over an entire large-area curved substrate. The concept of compensation described here is applicable in any generic optical system involved with illumination or imaging beams.
(2) Description of Related Art
Introduction to Optical Projection Lithography
In the recent past, electronics industry has witnessed dramatic increase in performance, throughput, yield and cost reduction with the advances in optical projection lithography. On the other hand, detector technology promises tremendous future for curved focal plane arrays (FPAs) in strategic and astronomical applications. Contact and non-contact projection lithography faces several challenges in patterning intricate details on curved surfaces. Anvik's systems are designed based on a novel, hexagonal seamless scanning concept and single-planar stage system configuration that provide both high optical and scanning efficiencies, and combine high-resolution imaging with very large exposure area capability. The prior art of Anvik's techniques for imaging on curved substrates has a curved mask that is identical in size and shape to the curved substrate for 1:1 patterning. There is a good description of curved-mask lithography in U.S. Pat. No. 6,416,908, PROJECTION LITHOGRAPHY ON CURVED SUBSTRATES, Klosner, Zemel, Jain & Farmiga, Jul. 9, 2002. However, a curved mask, because of its finite thickness, can cause several image anomalies due to its interaction with the illumination beam. In this invention, we propose and use a novel optical device, which we call ‘Zerogon’ that compensates for the image degradation associated with the use of curved masks.
Importance of the Illumination System
It has been a well-known fact from the times of invention of the microscope that the resolution and contrast of the microscope are significantly influenced by the technique of illumination of the sample. Similarly, the illumination technique can make a significant impact on the resolution and contrast of a lithographic projection system too. Though the illumination system is probably the most neglected or ignored part in such systems, some recent advances in illumination systems play great role in controlling the performance such as resolution, depth of focus and image contrast of a lithographic projection system. A few of these techniques are popularly known as off-axis illumination, annular source illumination, slit source illumination, 2-point source illumination, SHRINC illumination and use of phase shift masks.
Brief Review of Existing Illumination Techniques for Planar Masks
It has been an established fact that the use of curved Focal Plane Arrays (FPAs) can significantly influence the space and military applications in achieving wide fields-of-view for their sensors. Some of the techniques used for manufacturing these curved FPAs use curved masks in their projection systems. The several illumination techniques described above assume the use of planar masks in the object plane of the projection system. Use of curved masks in the object plane can cause severe image degradation due to defocus and beam deviations at the curved object plane. Problems associated with defocus of the condensed beam at the curved mask surface can be addressed by using special image motion compensating techniques within the condenser and the projection lens. On the other hand, beam deviations at the curved mask surface can significantly impact the light coupling between condenser and the projection lens affecting the partial coherence factor, which is the ratio of numerical apertures of condenser and the projection lens. A partial coherence factor value of 0.7 is normally chosen for incoherent illumination to achieve best resolution with projection lithography. In this paper, we describe a novel method to control the beam deviations at the curved mask plane, thereby protecting the partial coherence factor and the resolution characteristics of the imaging system.